Increasing demand for semiconductor memory and competitive pressures require higher density integrated circuit dynamic random access memories (DRAMs) based on one-transistor, one capacitor memory cells. But scaling down capacitors with the standard silicon oxide and nitride dielectric presents problems including decreasing the quantity of charge that may be stored in a cell. Consequently, alternative dielectrics with dielectric constants greater than those of silicon oxide and nitride are being investigated. Various dielectric materials are available, such as tantalum pentoxide, lead zirconate titanate, strontium bismuth tantalate, and barium strontium titanate (BST).
These alternative dielectrics are typically deposited at elevated temperatures and in an oxidizing ambient. As a result, an oxygen-stable bottom electrode material such as platinum or ruthenium oxide is used. Platinum, however, readily forms a silicide when in direct contact with silicon, and further is not a good barrier to oxygen due to fast diffusion down the platinum grain boundaries. A typical approach to solve this problem is to deposit the platinum on a thin oxidation barrier material. One such oxidation barrier material is titanium aluminum nitride. See co-pending, co-assigned application Texas Instruments Ser. No. 09/105,830, pending. See also Texas Instrument Ser. No. 09/105,738 now U.S. Pat. No. 6,153,490 and Ser. No. 09/05,411 now U.S. Pat. No. 6,090, for the hardmask and etch stop features of titanium aluminum nitride, or Ti-Al-N. Note that as used herein the notation "A-B-C" indicates that the material may exist in varying compositions of the elements A, B, and C, an example being "Si-O-N". An alternative notation is "Ti.sub.1-x Al.sub.x N," which is used herein in some instances.
Common deposition techniques for thin films include: physical vapor deposition (PVD), sputtering for example; thermal chemical vapor deposition (CVD); and plasma-enhance chemical vapor deposition (PECVD). PVD has been used for depositing Ti-Al-N, but lacks the film thickness control and step coverage available in CVD and PECVD systems. PECVD has been investigated for Ti-Al-N deposition. See for example S.-H. Lee, et al., "(Ti.sub.1-x Al.sub.x)N coatings by plasma-enhanced chemical vapor deposition," J. Vac. Sci. Technol. A 12(4), Jul/Aug 1994, p. 1602, and S.-H. Lee, et al., "Compositionally gradient (Ti1-xAlx)N coatings made by plasma enhanced chemical vapor deposition," J. Vac. Sci. Technol. A 13(4), Jul/Aug 1995, p. 2030. PECVD, however, also suffers from relatively poor step coverage, owing in large part to the directionality inherent in the electric field set up by the plasma.
Even though Ti-Al-N films produced by PVD and PECVD suffer from poor film quality and step coverage, no successful deposition of thermal CVD films has thus far been reported. The lack of a suitable precursor has been one obstacle to the implementation of CVD. The Lee papers cited above use inorganic precursors such as TiCl.sub.4, AlCl.sub.3, NH.sub.3, H.sub.2, and Ar in conjunction with PECVD for the deposition of Ti-Al-N. These precursors, however, typically require high temperatures to achieve deposition. In the Lee papers, for example, the reported substrate temperature is 450.degree. C. Thus, a need exists in the industry for a method of forming Ti-Al-N thin films with a thermal CVD process.